The invention relates to a device for axial stopping of a rotor. The device is provided for balancing machines capable of detecting unbalances in rotors occurring when the rotors themselves are driven in rotation.
The concerned rotors are preferably the armatures of electric motors used in micromotors or electric tools, or the armatures of electric motors of small and big household electrical appliances, electric motors employed in the automotive sector, etc.
It is known that rotors of electric motors are mounted on support shafts defining the rotation axis of said rotors and once mounted must be carefully measured and balanced, in order to prevent vibrations and unbalanced stresses to be generated in use, during rotation of said rotors, at a high number of revolutions per seconds.
The balancing machines carrying out measurement of these rotors or armatures comprise elements intended for support and control of the radial loads, and axial-abutment elements.
The elements intended for support and control of the radial loads, i.e. loads directed in a direction perpendicular to the rotation axis are generally fork-shaped elements supporting said shafts and connected to sensor devices capable of detecting and measuring the amount of possible unbalances, when a rotatory motion is imposed to the rotors.
The axial-abutment elements are on the contrary designed to abut against the opposite end faces of the rotor shafts, to hold them in an axial direction, while the rotatory motion necessary to detect and measure the unbalances to which the rotors are subjected is being transmitted to the rotors.
Practically in the known art the axial-abutment elements are generally made up of a pair of elastic foils held contact, at their substantially flat portions, with said end faces, against which they slide during the rotatory motion of the rotors.
It should be pointed out that in balancing machines the rotatory motion is transmitted to the rotors through belts of a material having a high friction coefficient the belts being in contact with the outer surface of the rotors themselves when their support shafts are caused to rest on said fork-shaped support elements.
To avoid occurrence of axial oscillations, during the rotor rotations, between the abutment elements defined by said elastic foils, the rotors are such disposed that their rotation axis is not perfectly perpendicular to said rotation-controlling belts.
In fact a slightly inclined orientation of the rotors relative to said belts is generally provided so that the belts may generate dragging forces on the rotors having a small component directed towards one of the two axial-abutment elements. Consequently, each rotor is pushed with a reduced force in one way alone of its axial direction and only one of the two axial-abutment elements must react to the action exerted by the corresponding end face of the shaft. The other axial-abutment element exclusively performs a safety function, but practically is not submitted to any stress.
In this manner, a good stability of the axial position of the rotors is obtained. The known art briefly described above is satisfactory in many cases, but it has the drawback that sometimes it is not adapted to reach the highest reliability qualities required by the most severe standards concerning balancing machines. In particular, the requirement of a great precision is felt, in order to achieve repeatability and reliability in measures or xe2x80x9ccapabilityxe2x80x9d, in the cases in which for example it is provided that measures repeated a great number of times on the same rotor or armature might differ by ten per cent at the most from each other with respect to the proper tolerance value of the balancing machines which is already very small by itself.
From experimental tests it came out that unevennesses in the measurement results of rotor unbalances arise at least partly at the axial abutment elements used in the known art to axially retain the rotors, and that unbalances depend on the finish degree of the ends of the rotor-supporting shafts.
In fact, the end faces of the rotor supporting shafts have an imperfect perpendicularity relative to the rotation axis and this imperfect perpendicularity gives rise, on sliding against the axial-abutment elements i.e. the elastic foils, to additional vibrations that adversely affect the whole rotor thereby modifying detection from the balancing machines.
The above mentioned additional vibrations can also result from geometric and/or positioning defects between the support shafts and the axial-abutment elements. Practically, it was found out that due to small imperfections at the axial-abutment elements, vibrations are generated that are interpreted as unbalance signals, which unbalances do not in fact exist.
In order to overcome the above mentioned drawback, a reduction in the perpendicularity error of the end faces of the rotor-supporting shafts cannot be in any case envisaged, nor can a raising of the working level of the shafts be conceived.
In fact this technical solution would involve a heavy rise in the production costs of the rotors in a portion thereof that is not of great importance as regards operation. In addition this technical solution would not enable complete cancellation of the influence exerted by said vibrations on the unbalance measure, due to contact of said end faces against the axial-abutment elements.
Under this situation the technical task underlying the invention is to conceive a device for axial stopping of rotor balancing machines capable of substantially obviating the mentioned drawbacks.
Within the scope of this technical task, it is an important aim of the invention to provide an axial-stopping device capable of preventing that, in measurements, other periodic unevennesses resulting from axial engagement of the support shaft with the axial- abutment elements should be added to unbalances typical of rotors.
The technical task mentioned and the aims specified are achieved by a device for axial stopping of a rotor, in particular an armature of an electric motor, for balancing machines, said rotor having a support shaft defining a rotation axis and two end faces transverse to said rotation axis, said device comprising at least one thrust unit having an abutment surface adjacent to one of said end face and adapted to exert a repulsive force on said end face able to axially stop said support shaft and to keep an interstice between said abutment surface and said end face of said support shaft.